Two things are needed for combustion in internal combustion engines: air and fuel. On the air side of the equation, however, we’re really talking about the oxygen in the air around us. Oxygen is the specific element needed to combine with the fuel for burning, and it’s that burning — combustion — that creates the power needed to drive the engine.

You know what’s not needed for combustion? Any kind of dirt, debris, or other contaminants that may be floating around with that oxygen. An air filter’s job is to trap that kind of stuff as it comes through the air intake, allowing only clean air to pass by and keeping harmful materials from getting into the engine. To do that, air filters come in two basic setups: dry or wet.

The first relies on, obviously, dry filter material that often looks like paper or fabric to keep dirt and whatnot from entering the engine. The latter wears an additional thin coating of oil that helps trap even finer particles from passing by. The key real-world difference is that dry filters are meant to be disposable and replaced when they get too dirty, while wet filters can be cleaned and reused.